The second link is useful but how to get the 'diagrams package' (so that \begin{diagram} can be used)? It doesn't seem to be on the website.

I have never used the package myself...
but following the description in that pdf,
it says...
"The diagrams package -
This is not part of the standard MikTex installation, but you can get it from the
author’s home page http://www.cs.man.ac.uk/~pt/diagrams/."
That link leads tohttp://www.cs.man.ac.uk/~pt/diagrams/ which eventually redirects to http://www.monad.me.uk/diagrams/
which links to a page that says
"Unlike many other packages there is no "installation procedure" - you just copy the macros themselves to your TEX macros directory" with a link to http://www.monad.me.uk/diagrams/diagrams.sty .
Try that out and let us know if it works.

I have never used the package myself...
but following the description in that pdf,
it says...
"The diagrams package -
This is not part of the standard MikTex installation, but you can get it from the
author’s home page http://www.cs.man.ac.uk/~pt/diagrams/."
That link leads tohttp://www.cs.man.ac.uk/~pt/diagrams/ which eventually redirects to http://www.monad.me.uk/diagrams/
which links to a page that says
"Unlike many other packages there is no "installation procedure" - you just copy the macros themselves to your TEX macros directory" with a link to http://www.monad.me.uk/diagrams/diagrams.sty .
Try that out and let us know if it works.

Macros are useful user-defined abbreviations of complicated commands.
Many are conveniently wrapped into style files or packages.

Style files can be used by placing the file in the same directory as the .tex source.
For a more centralized location, there is probably a directory in your tex distribution with lots of .sty files in it.

Macros are useful user-defined abbreviations of complicated commands.
Many are conveniently wrapped into style files or packages.

Style files can be used by placing the file in the same directory as the .tex source.
For a more centralized location, there is probably a directory in your tex distribution with lots of .sty files in it.